Military weapons systems and automotive air bag systems are typically activated by an electroexplosive device. The EED usually employs a small metal bridgewire to ignite a contained explosive mixture. An electric current typically in the range of from about 1 amp to about 7 amps is passed through the bridgewire. Internal resistance heats the bridgewire to a temperature in excess of about 900.degree. K. The hot bridgewire ignites an energetic powder, triggering the primer which in turn ignites the propellant or explosive in the system. The system may incorporate a pyrotechnic mixture, a propellant or an explosive powder.
A problem with the bridgewire type EED is a sensitivity to externally generated electric currents. High levels of electromagnetic energy from sources such as radio waves, static electricity, lightning or radar may induce an electric current within the bridgewire sufficient to cause an undesired, premature ignition.
The invention of the semiconductor bridge for electroexplosive devices was disclosed in U.S. Pat. No. 3,366,055 by Hollander, Jr. Several embodiments were described by Hollander which encompass all current materials used to fabricate SCBs. A semiconductor bridge circuit as described by Hollander, Jr. will initiate the explosive reaction within the primer when a current is applied. The SCB circuit is significantly less susceptible to induced electric currents and the resultant possibility of accidental or premature ignition is reduced.
A semiconductor bridge circuit comprises a circuit formed on a semiconductor material such as silicon. A heavily doped silicon region of an n-type dopant such as phosphorous is vaporized when a current of sufficient amperage is applied. The silicon vapor is electrically heated and permeates the adjacent energetic powder mixture. Through localized convection and condensation, the energetic powder is heated to its ignition temperature leading to the desired explosive reaction being initiated.
FIG. 1 shows in cross-sectional representation an EED 10 for a semiconductor bridge circuit 12 as known in the prior art. The housing 20 encases a semiconductor device 12 formed from a semiconductor material such as silicon. The SCB device includes a heavily doped bridge 13 which vaporizes when a threshold current is applied. The primer housing 20 positions the bridge 13 in close proximity to a charge 14 of an energetic powder such as lead azide The EED 10 comprises a pair of metallic feed through leads 16 which pass through a ceramic header 18. A conventional glass to metal seal bonds the feed through leads 16 to the header 18. A metallic casing 20 made, for example, of aluminum surrounds the ceramic header 18 and a charge holder 22. Wire bonds 24 electrically interconnect the metal feed through leads 16 to bond pads 26 formed on opposite sides of the surface of the semiconductor bridge device 12, with one bonding pad located on each side of the bridge and connecting to the lead wire on the surface of the die. When a voltage is applied across feed through leads 16, current flows through the bridge 13. The bridge vaporizes forming a plasma cloud within the energetic powder 14. The electric current further heats the plasma vapor such that local convection and condensation heat the energetic powder 14 to ignition. The entire process from application of voltage to ignition takes place in less than about 20 micro-seconds.
A problem with the primer housing 10 of the prior art are (1) the ceramic header 18 is brittle and subject to fracture when the explosive device is handled roughly, and (2) the wire bonds 24 are in contact with the primer charge 14. The primer charge is compacted to maximize the explosive energy. Another problem is that compaction of the powder 14 applies stresses to the wire bonds 24 potentially leading to the wires either breaking or pulling loose from either the feed through leads 16 or from the bond pads 26. This package is not a preferred structure. Forming ceramic headers with metal feed-throughs is a relatively expensive process adding to the cost of the device. This is particularly true if the casing 20 must be hermetically sealed against the ceramic 18. Further, if large electrical pads are used to achieve low resistance connections, it increases the die 12 area and therefore the size and cost of the device.
The advantages of the SCB type initiator over the bridgewire include lower electrical energy requirements, less susceptibility to accidental or premature initiation and more rapid and precise firing times. However, methods used to attach the semiconductor bridge die to the EED header have demonstrated poor reliability and have been costly to produce. The SCB circuit is formed on a brittle semiconductor substrate. The package housing the device must provide both mechanical and environmental protection to the device. The components making up the electronic package must also be compatible with the SCB device, the energetic powder, and the attachment materials. The electrical connections to the die must withstand pressure from powder loading and consolidation.
Several patents have focused on methods for attaching the SCB to a header in order to lower cost and improve reliability. One method for fabricating the SCB to achieve efficient attachment to a header is disclosed in U.S. Pat. No. 4,708,069 to Bickes, Jr., et al., and in Sandia National Labs Report No. SAND 86-2211 edited by Bickes, Jr., both of which are incorporated herein by reference. Bickes is distinguished from Hollander by using "a pair of spaced pads connected by a bridge, the area of each of said pads being much larger than the area of said bridge . . . " as shown in FIG. 2. These large pads 30 are used to achieve electrical contact with a metallized layer 34 covering the pads. The large pad size described in Bickes was used to achieve a low resistance connection to the polysilicon bridge material 32. This low resistance contact allowed a low impedance bridge, typically about 1 ohm which is common in the art, to be used which further reduced susceptibility to RF energy.
Subsequently, in U.S. Pat. No. 5,029,529, Mandigo discloses a method of attaching an SCB die in an electrical primer housing which eliminates one lead wire to the die (FIG. 3) which is an apparent improvement over Bickes, et al. An electrically conducting die attach means 72 is used to attach the SCB die 52 to a copper alloy primer button 40. The electrical pulse to fire the bridge follows a conductive path 74 through the silicon based device 52 and through a conductor or shunt 76 attached to the side of the die 52, The attachment method disclosed by Mandigo was used with an electrical primer 70 which is constructed from a cup 42 which forms one electrode for the external current source and a button 40 which forms the second electrode. This configuration requires an interposed insulator 54 and a conducting path from the cup 42 to the die 52 created by a wire 44 attached to the die and a conducting element 48 which is attached to the cup. This application requires a more complex assembly than conventional EEDs because it is used in gun ammunition; it is relevant to this disclosure only because of the method of achieving a conducting path through the silicon die 74 by doping the silicon. The attachment method of Mandigo suffers from three disadvantages. First, the shunt 76 must be attached on the side of the die after the die is cut from the wafer; this process is not easily performed with standard semiconductor processing technology. Second, a single wire 44 connects the bridge to the conducting case, which is subject to failure. Third, the method utilizes the large pads of Bickes, et al. with the attendant disadvantages discussed above.